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Is it possible they have not been assigned a pagerank? I was under the impression it would be a "greyed" out ranking instead.
There is at least one internal page that has a rank of 0, yet ranks #4 for its keyword, so I'm confused.
The common understanding of Page Rank that you can pick up around the web is almost always off just a bit, and sometimes it's off in very important ways. There's some very helpful information around the forum here, starting with the thread that's in our Hot Topics [webmasterworld.com] thread, pinned to the top of this forum's index page.
Google PR - PageRank FAQs [webmasterworld.com]
A related question - while a site gains trust / ranking through age related factors - does this apply equally to individual page url locations? ie will the shift to the new url structure "reset the counter" on each individual page - do individual pages carry age/trust factors?
My own preference when changing urls is to use a light touch on 301s - studying the site's logs and backlinks and then only placing a 301 redirect on the key legacy urls. So the 301 only goes to urls with good external backlinks or heavy search engine traffic. The rest of them I would rather see return a 404. Internal backlinks don't matter as those links are being changes on the page anyway.
This approach has been quite successful to date, and where I've used it there has been no dreaded dip in traffic when the rewritten url structure was launched.